Purpose or Case Report: Although juvenile inflammatory arthritis is one of the most common atraumatic joint pathologies in children, many other infectious, vascular, and hemorrhagic lesions, benign and malignant neoplasms, and tumor-like conditions are known to affect the pediatric synovium and periarticular soft tissues. In a child with suspected joint pathology, radiographs are an essential part of the initial imaging assessment, followed by MRI for more definitive characterization of the synovial lesion. Ultrasound is also an important adjunct modality in the imaging of these entities.

In this educational poster, we highlight the diagnostic imaging features of some of the common and uncommon pediatric synovial pathologies with emphasis on the specific MRI characteristics that aid in accurate diagnoses, such as presence of a solid or cystic mass, thick enhancing synovium, rice bodies, cartilaginous and osseous loose bodies, vascular channels, hemosiderin staining, articular erosions and marrow edema. Synovial sarcoma and vascular malformations, while not truly synovial lesions, are also considered because they are sometimes located near the joints and because they can have clinical and imaging overlap with synovial lesions.

The following entities will be illustrated and reviewed:
1. Baker cyst
2. Ganglion cyst
3. Oligo-articular JIA (knee and wrist)
4. Tuberculous arthritis
5. Tenosynovial giant cell tumor
6. Hemophilic arthropathy
7. Synovial chondromatosis
8. Low-flow vascular malformation
9. Synovial sarcoma (knee, ankle and elbow)

We also propose an MRI-based diagnostic algorithm for accurate characterization of pediatric synovial pathologies based on morphology (localized versus diffuse, cystic versus solid); signal characteristics on T2-weighted images, gradient sequences and post-gadolinium images; and associated osseous changes, in addition to certain highly specific radiological features of each entity.
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Results:
Conclusions: Imaging plays a critical role in characterization of synovial pathologies and may preclude the need for invasive diagnostic arthroscopy. Recognition and systematic assessment of the imaging features of these lesions may aid diagnostic accuracy and help to guide management decisions.